The Mediterranean Sea presents several biogeochemical anomalies compared to the global ocean. An unbalanced N budget, high nitrate/phosphate ratios in subsurface waters and low N-15/N-14 ratios in particulate and dissolved nitrogen suggest a significant occurrence of N-2 fixation. This study presents, for the first time, a basin-wide overview of direct measurements of N-2 fixation, with values in the North Atlantic for comparison, during late spring 2007. Very low N-2 fixation rates (0.052 +/- 0.031 nmols N l(-1) d(-1)) were observed in all sub-regions of the Mediterranean, in contrast to the higher values measured in the North Atlantic (0.300 +/- 0.115 nmols N l(-1) d(-1)). Higher phosphorus (inorganic or organic) concentrations were not associated with higher N-2 fixation rates. Low N-15/N-14 ratios in particulate organic nitrogen (from -2.10 to +4.11%), associated with low N-2 fixation rates, suggest that other N sources, such as atmospheric inputs, fuel the Mediterranean ecosystem. Citation: Ibello, V., C. Cantoni, S. Cozzi, and G. Civitarese (2010), First basin - wide experimental results on N-2 fixation in the open Mediterranean Sea, Geophys. Res. Lett., 37, L03608, doi: 10.1029/2009GL041635.